The invention relates to a process for making indole.
Indole is a valuable material in the perfume industry and is used for the fixation of perfuming agents. It is also the starting material for the synthesis of the growth regulator indolyl-3-acetic acid. Indole is furthermore used for making the essential amino acid L-tryptophane (.beta.-indolyl-alanine) which when added to animal feed in small amounts contributes to an increase of the meat production.
Indole is found in coal tar in an amount of only about 0.2%. Its separation from the materials present with it is difficult and results in losses of substance. The amount of indole which can be obtained from coal tar is insufficient to meet the increasing demands.
Indole can also be synthesized in different ways. Most of the prior art processes proceed from o-ethylaniline from which indole is obtained by dehydrocyclization in the gas phase. In this process o-ethylaniline is evaporated and subjected to a heat treatment at 400.degree. to 700.degree. C. in the presence of an inert carrier gas such as hydrogen, nitrogen or also steam, together with a dehydrogenation catalyst. Suitable catalysts are Al.sub.2 O.sub.3 or SiO.sub.2 onto which heavy metal oxides such as chromium, copper, molybdenum or vanadium oxides are deposited or the catalyst consists of precious metal carrier catalysts containing platinum or palladium. The conversion of the o-ethylaniline and the yield of indole depends greatly on the reaction conditions and the composition of the catalyst. The conversion usually is between 40 and 60%. The yield related to the converted o-ethyl-aniline is between 20 and 60% of the theoretical value. There have, however, already been obtained yields of above 80% of the theoretical amount. Byproducts of the prior art processes are aniline, o-toluidine and o-aminostyrene (West German accepted applications Nos. 22 24 556 and 24 01 017.)
The shortcomings of the processes proceeding from o-ethylaniline are caused by the fact that the making of the o-ehtylaniline by nitration of ethylbenzene and subsequent reduction of the nitro group involves substantial losses. Besides, the activity of the dehydrogenation catalysts decreases rapidly.
It has also become known to make indole from N-(.beta.-hydroxyethyl)-aniline by esterifying the alcohol function of the N-(.beta.-hydroxyethyl)-aniline in an organic environment with an inorganic or organic acid at a temperature between 0.degree. and 200.degree. C. in a first step, and then subjecting the thus formed ester in a second step at a temperature between 650.degree. and 900.degree. C. to a thermal tracking treatment (West German accepted application No. 23 28 284). This prior art process is also difficult and complex like the processes starting from o-ethylaniline because it involves several reaction and conversion steps.
The present invention therefore has the object to improve the availability of indole by a process which has a high yield and starts from a readily available starting product.